This invention relates to optics, and more particularly to catoptric imaging systems.
Imaging systems of varying sorts have existed for thousands of years. Even after such a long period of development, modern imaging systems still have a similar purpose as their ancient counterparts. Imaging systems gather light from an object point and its vicinity and focus this light into an image at an image point and its vicinity. Light can be focused using refraction and this branch of optics is known as dioptrics. Light can also be focused using reflection and this branch of optics is known as catoptrics. Irregardless of the focusing system used typical imaging systems strive to optimize a few important parameters. For example, many imaging systems are designed to optimize resolution, numerical aperture, and shape of the image plane. Resolution of the imaging system is the smallest distance between features in the object space that can be distinguished in the image plane. Therefore resolution determines the level of detail that can be derived from the image. Numerical aperture relates to the amount of available light that the imaging system collects from the object. For most types of detectors, such as photographic film, charge coupled devices, or even the human eye, a larger numerical aperture increases light intensity and typically yields better images. Lastly, the shape of the image plane can be quite important. A flat plane is typically most useful for detection devices like photographic film or charged coupled devices (CCD). Unfortunately, all of these parameters can be degraded by a host of aberrations.
The present invention features catoptric optical systems that utilize a beam splitter surface and a reflecting surface. Primary focusing can be achieved with the reflecting surface and therefore longitudinal chromatic aberrations are reduced. The beam splitter is positioned relative to the object point, image point, and the reflective surface such that light rays from the object point which are focused to the image point have been both reflected and transmitted by the beam splitter surface. The combination of a reflection and a transmission for each ray of the beams being focused substantially eliminates first-order variations in the beam intensity due to imperfections in the reflective and transmissive properties of the beam splitter for incident angles deviating from a central design angle. In some embodiments of the system, light transmission may be enhanced by use of interferometric recombination of light reflected and transmitted by the beam splitter. Furthermore, some embodiments of the system may include refractive elements to reduce aberrations.
In general, in one aspect, the invention features an imaging system for imaging an object point to an image point. The system includes i) a beam splitter positioned to receive light rays from the object point and separate each ray into a transmitted portion and a reflected portion, the transmitted portions defining a first set of rays and the reflected portions defining a second set of rays; and ii) a reflecting surface positioned to receive one of the sets of rays from the beam splitter and focus that set of rays towards the image point via the beam splitter.
Embodiments of the imaging system may include any of the following features.
The reflecting surface may be positioned to receive the first set of rays and reflect the first set of rays back to the beam splitter, in which case the beam splitter is positioned to reflect at least a portion of each ray received from the reflecting surface to the image point. Furthermore, the reflecting surface may be substantially concentric with the object point. A center of the reflecting surface may define an object optical axis with the object point, and the beam splitter may be positioned substantially perpendicular to the object optical axis or at an acute angle to the object optical axis (e.g., an acute angle substantially equal to 45 degrees).
Alternatively, the reflecting surface may be positioned to receive the second set of rays and reflect the second set of rays back to the beam splitter, in which case the beam splitter is positioned to transmit at least a portion of each ray received from the reflecting surface to the image point. Furthermore, the reflecting surface may be substantially concentric with the image point. A center of the reflecting surface may define an image optical axis with the image point, and the beam splitter may be positioned substantially perpendicular to the image optical axis or at an acute angle to the image optical axis (e.g., an acute angle substantially equal to 45 degrees).
The imaging system may further include a first optic having an internal surface defining the reflecting surface. For example, the internal surface of the first optic may be curved. The first optic may have a flat surface opposite the internal surface, and the beam splitter may be positioned adjacent the flat surface. Furthermore, the system may include a plano-convex optic having a piano surface adjacent one of the object point and the image point and a convex surface contacting the first optic, wherein the interface between the plano-convex optic and the first optic defines a refracting surface.
More generally, the imaging system may include a refracting surface positioned between the object point and the beam splitter to receive the light rays from the object point. For example, the refracting surface may be substantially concentric with the object point. Alternatively, or in addition, the system may include a refracting surface positioned between the beam splitter and the image point to receive the light rays focused by the reflecting surface. For example, the refracting surface may substantially concentric with the image point.
The system may also include a second optic adjacent the first optic, wherein the beam splitter is positioned at an interface between the first and second reflective optics. Furthermore, the system may include a plano-convex optic having a piano surface adjacent one of the object point and the image point and a convex surface contacting the second optic, wherein the interface between the plano-convex optic and the second optic defines a refracting surface. Moreover, the system may further include another plano-convex optic having a piano surface adjacent the other of the object point and the image point and a convex surface contacting the first optic, wherein the interface between the plano-convex optic and the first optic defines another refracting surface.
The second optic may be an optical flat. Alternatively, the second optic may have an internal surface defining a second reflecting surface, and wherein the first reflecting surface is positioned to receive the first set of rays and the second reflecting surface is positioned to received the second set of rays and focus them towards the image point via the beam splitter. Furthermore, the beam splitter may be positioned to interferometrically recombine the first set of rays received from the first reflecting surface and the second set of rays received from the second reflecting surface. The first reflecting surface may be concentric with the object point and the second reflecting surface may be concentric with the image point.
In general, in another aspect, the invention features an imaging system including a first imaging subsystem for imaging an object point to a first image point; and a second imaging subsystem for imaging the first image point to a second image point. The first imaging subsystem second includes i) a first beam splitter positioned to receive light rays from the object point and separate each ray into a transmitted portion and a reflected portion, wherein the transmitted portions define a first set of rays and the reflected portions defining a second set of rays; and ii) a first reflecting surface positioned to receive one of the sets of rays from the first beam splitter and focus that set of rays towards the first image point via the first beam splitter. The second imaging subsystem includes (i) a second beam splitter positioned to receive light rays from the first image point and separate each ray into a transmitted portion and a reflected portion, wherein the transmitted portions define a first set of rays and the reflected portions defining a second set of rays; and (ii) a second reflecting surface positioned to receive one of the sets of rays from the second beam splitter and focus that set of rays towards the second image point via the second beam splitter.
In general, in another aspect, the invention features an imaging method for imaging an object point to an image point. The method including: receiving light rays from the object point and separating each ray into a transmitted portion and a reflected portion, the transmitted portions defining a first set of rays and the reflected portions defining a second set of rays; and receiving one of the sets of rays from the beam splitter and reflecting that set of rays to focus them towards the image point via the beam splitter.
Embodiments of the inventions may include any of the following advantages.
They may have a large numerical aperture in the object space. They may have substantially no longitudinal chromatic aberrations. They may have an image plane whose location is substantially independent of the spectral region used in image formation. They may use a large spectral range for image formation. They may have a flat image plane. They may have a reduced set of optical aberrations. They may have a magnification which is less than, greater than, or equal to one.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.